If you're talking about models, like say, consoles, statues or things like that. Those are usually made in a 3d modeling program and then exported into a format jka/gtkradian uses. you can also compile models from brush/patchwork in gtkradiant. If you're just talking about the regular curvy parts of the map, you're going to have to build those in gtkradiant. Anything curved needs to be built with patches. The rest of the squared off stuff is built with brushes. If you could prove a screenshot of what you want to recreate someone may be able to help you more.

l will try to put a screenshot if l can that is but thank's for the help l realy needed it thank's